1. Field of the invention
The present invention relates generally to optical wavelength converters and, more specifically, to a wavelength converter that receives an optical input at an arbitrary wavelength and produces an optical output at a designated wavelength.
2. Description of the Related Art
Data transmission technology is currently undergoing the dramatic change from electrical signal-based transmission to optical signal-based transmission. The optical revolution is providing high data transmission rates using inexpensive, reliable devices. A key advantage of optical signal transmission is the ability of a single transmission line, an optical fiber, to carry a high number of optical signals at different wavelengths simultaneously without interference among the signals. Thus, a single optical fiber may carry simultaneously many xe2x80x9cchannelsxe2x80x9d of communication. Several wavelength ranges, called xe2x80x9cbandsxe2x80x9d, are currently widely used. The most promising bands are the xe2x80x9cCxe2x80x9d and xe2x80x9cLxe2x80x9d bands at 1520-1565 nanometers (nm) and 1565 to 1625 nm, respectively, due to the low absorption and dispersion of signals transmitted at wavelengths within these bands through currently available optical fiber.
A functional wide-area optical network exists as a connected set of distributed routing and switching nodes. User equipment may be connected to these nodes to receive and transmit data. Many communications must be transmitted simultaneously through a network. It is not feasible to permanently or globally allocate unique wavelengths to each user or particular node-to-node network connection. A flexible networking strategy is preferred which can tentatively and locally allocate a wavelength xe2x80x9cchannelxe2x80x9d to a particular data transmission. This allows a particular data transmission to traverse a network utilizing immediately and locally available channels instead of being delayed until a particular channel is globally open. Such flexibility limits the number of necessary transmission lines and the costs thereof. This strategy requires that a data transmission initiated at one wavelength be seamlessly converted where necessary to another wavelength. Optimal flexibility will include intraband and interband conversion. Intraband conversion occurs when a signal of an initial wavelength is converted to a similar final wavelength such that the initial and final wavelengths lie together in a band, for example, the C band. Interband conversion occurs when the initial and final wavelengths are dissimilar such that they lie in different bands, for example, a C band signal may be converted to an L band signal.
Non-linear optical (NLO) materials which have crystalline structures that exhibit non-zero second-order nonlinear electric susceptibilities ("khgr"(2)) are now available offering efficient optical frequency conversion. Supported within such materials are three wave mixing (TWM) processes whereby fundamental, second, and higher harmonic wave modes interact within the crystal so that optical energy is transferred among modes. Resulting, are such known processes as second harmonic generation (SHG), and difference frequency generation (DFG). SHG can be understood as the interaction of two pump photons resulting in a generated photon of twice the energy of a pump photon. A similar process, sum frequency generation (SFG), can be understood as the interaction of a pump photon and a signal photon resulting in a generated photon with a frequency which is the sum of the frequencies of the pump photon and signal photon. DFG can be understood as the interaction of a pump photon and a signal photon resulting in a generated photon with a frequency which is the difference of the frequencies of the pump photon and signal photon.
For efficient frequency conversion, the interacting waves of different frequencies must maintain a coherent phase relationship as they propagate the interaction length of the process supporting crystal. This is because waves of dissimilar frequency propagate along the interaction path at dissimilar velocities and so become gradually out of phase. A successful strategy for maintaining phase matching is called quasi-phase matching (QPM). It involves periodic modulation of the refractive index along the interaction length such that the harmonic fields remain in phase at the beginning of each period. A microdomain periodicity can be produced within a crystal using spatially alternating electric fields or periodic ion exchange or implantation along the axis of the interaction length in the process of manufacturing the crystal. Phase maintenance results from choosing an NLO crystal with microdomain periodicity to match the fundamental or harmonic wave to be produced in a TWM process.
Direct optical signal amplification and wavelength conversion will soon eliminate the high costs, and slow processing of optical-electro-optical (OEO) devices. OEO wavelength converters known in the art lack sufficient bandwidth to accommodate the very high switching speeds of optical networks currently under development. Therefore, there is a need for a high-bandwidth optically transparent wavelength converter utilizing direct optical-optical conversion.
The present invention relates to an optical wavelength converter that includes an optical sum frequency generator (SFG) and an optical difference frequency generator (DFG). The SFG receives part of both an input beam and a continuous-wave (CW) beam. The DFG receives part of the input beam as well as the output of the SFG. The output of the DFG represents the signal of the input beam modulated or carried on a beam having the frequency of the CW beam.